An impeller incorporating these features is already known from FR-PS 1 326 701. This impeller is particularly provided to accomplish an improved pressure stability in dependence on the flow rate. The essential feature of the impeller is that the efflux surfaces of the blades have trailing edges curving inwardly in opposite direction. Impeller blades of such a profile produce a stable air stream on their trailing edges along the entire zone of pressure flow of the blower, in contrast to an unstable flow developing in blowers with a conventional blade profile in the presence of specific pressure relations and specific amounts of flow. Preferably, the trailing edges of the blades are arranged in parallel with the leading edges of the blades, satisfactory results being obtained also if the alignment of the leading edges departs from that of the trailing edges by as much as 15 degrees. The curvature in opposite direction of the trailing edges of the blades relating only to the last fourth of the efflux surfaces, it may nevertheless be advantageous in certain cases that this curvature in opposite direction begins directly after the area between the trailing and leading edge of the blades. The rotors may be integrally manufactured by casting, injection-molding or milling, and may be made of an aluminum alloy, stainless alloyed steel, plastic, or a similar material.
The teaching is essentially aimed at obtaining a blade geometry such that the direction vector of the leading and trailing edge of each blade extends largely in parallel. The design method selected for this purpose consists in projecting various profiles on concentric circles. This results in a type of surface for the description of the blade geometry which cannot be described by rulers. Generating and making a pattern of such a type of surface incurs increased expenditure in all stages of the design and manufacture of such an impeller, starting with the determination of the surface points in the design stage up to the manufacture of the tools or injection molds. Moreover, the design method as it appears from FR-PS 1 326 701 does not ensure that an impeller with a predetermined blade profile is in fact ejectable from the mold, that is, it does not ensure that a slide of the injection mold inserted between two blades is movable radially to the axis of rotation of the impeller without a collision occurring. Overall, FR-PS 1 326 701 does not suggest a design for the manufacture of an impeller in which it is ensured that the impeller is capable of being manufactured by injection molding in addition to being ejectable from the mold, with the individual blade profile being practically exclusively determined by fluidic or physical, rather than manufacturing or tool related, side constraints.
A further impeller is known from U.S. Pat. No. 4,678,410. The impellers described in this prior publication have shown excellent results in practice on account of their high efficiency, their particular engineering design lending itself to large-scale production applying injection-molding processes. It is especially this last-mentioned advantage of the known impeller which is remarkable in so far as the problem of ejecting such impellers from the mold has been solved in a simple manner, in spite of undercuts existing between the blades and the hub structure, by using an injection mold of a straightforward structure. However . the blades of the known impeller are shaped and arranged on the circumferential outer surf ace of the hub structure such that there is no overlap of adjacent blades. A gap is invariably maintained between adjacent blades.
In the presence of this known and already largely optimized impeller, it is nevertheless desirable for a variety of reasons to provide an impeller with overlapping blades. To begin with, the efficiency of an impeller can be further increased by having blades of a suitable profile overlap each other. In the second place, the use of impellers having overlapping blades makes specific supplementary structures redundant which otherwise would be necessary to protect the user. Thus, the use of an impeller with overlapping blades obviates the provision of supplementary structures, as for preventing the user from accidentally reaching through the impeller with thin metallic objects into the interior of the axial-flow fan or the appliance for drying and dressing hair, thereby eliminating the danger of contact with live parts inside the axial-flow fan or the hair dryer. Moreover, the design methods hitherto known and the methods derived therefrom for producing test specimens and injection molds have not been perfected to a degree ensuring ejection of an impeller with overlapping blades from the mold, that is, its manufacture by injection molding has not been technically feasible. The absolutely necessary prerequisites for a manufacture of the impellers by injection molding, such as the ejective capability, a realistic slide geometry and slide kinematics of the injection mold, presented material difficulties in the manufacture of such an impeller as an injection-molded plastic part, in addition to imposing restrictions on a blade profile affording optimum fluidic conditions.